There is no simple prior art for measuring an angle in the plane containing the observer, hereafter referred to as the "yaw" angle. In missile testing this is one of the principal variables of interest. One current technique is to hoist a camera far above the missile launch, looking downward. Then the viewing perspective is perpendicular to the angle of interest, and it can be measured with a protractor on the film or video tape. This is restricted to the launch phase of the flight.
Another known invention utilizes a pair of laser trackers illuminating roof reflectors on the missile, which must be rolling, and determines the yaw angle from the differential time of arrival of the signal at different sites. While this idea works in principal, the cost of multiple tracking stations has kept implementation out of reach.
It may be possible to determine the yaw angle by continuous doppler measurement of retroreflectors placed on different part of the missile. Again, they require instrumentation more complex than is normally found on the range, and measure angular rate, rather than position. Therefore a continous track is necessary to deduce angular position.